146 PHYSIOLOGICAL BOTANY. 



CHAPTER IX. 



ABSORPTION. 



460. Office of the root. The absorption of liquids, con 

 taining in solution the food of the plant, is the peculiar and 

 indispensable office of the root. 



Take a small growing plant from the earth and immerse it by its roots, which should 

 be nearly or quite entire, in a cup containing a definite quantity of water. Place near it 

 another cup with a like quantity of water to indicate the amount of evaporation. The 

 difference of the diminution in the two cups will be the amount of absorption. A plant 

 of Spearmint has thus been found to absorb more than twice its own weight per day. 

 Every one is familiar with the rapid disappearance of water from the roots of potted 

 plants, as Hydrangea, Oleander. 



461. An impervious epidermis destitute of stomata every- 

 where clothes the roots, excepting its fibrillce and the tender 

 extremities of the rootlets. No part, therefore, is capable of 

 absorption except the latter. But these, by their multiplied 

 numbers, present to the soil an adequate absorbing surface. 

 Hence, in transplanting a tree, almost the only danger to its life 

 arises from the difficulty of preserving a sufficient number of 

 these rootlets. 



462. The force with which plants absorb fluids by their roots ia surprisingly great. If 

 the stem of a Grapevine be cut off when the sap is ascending, and a bladder be tied to 

 the end of the standing part, it will in a few days become distended with sap even to 

 bursting. DR. HALES contrived to fix a mercurial gauge to a vine thus severed, and found 

 the upward pressure of the sap equal to twenty-six inches of mercury, or thirteen pounds 

 to the square inch. 



463. But what causes this absorption of fluids in a direction 



contrary to gravitation ? In explanation of this phenomenon 



reference has been made to two well-known principles in physics 



viz., to capillary attraction by the tubular vessels, and to en- 



. dosmose by the closed cells, which are far more numerous. 



464. The effects of capillary attraction are seen in experiments like the following. In- 1 

 eert the end of several open thermometer tubes in a colored liquid. It will be seen rising 

 in the tubes above its level to various heights highest in the smallest calibre. Or sus- 

 pend a napkin in such fashion that its lowest corner shall dip into a cup of water. In a 

 few hours the water will have ascended into the napkin. 



4tS. Endosmose is thus illustrated: Throw dried prunes, currants, or raisins into 

 water. After a while they will have become swollen and distended with fluid. Now place 

 them in strong syrup they will again shrink. The flowing in, as in the first case, is called 

 endosmose ; the flowing out, as in the second case, is called exosmose. (See Class Book of 

 Botany, 781-783.) Another explanation of absorption is given in 471. 



